1. Field of the Invention
The present invention relates to a developing unit and an image forming apparatus, and more particularly, to a developer collecting mechanism.
2. Description of the Related Art
In an image forming apparatus, such as a copier, a printer, a facsimile apparatus, or a printing machine, a copy of an image is obtained by transferring a visible image formed on a photo-sensitive element as a latent image onto a recording medium, such as a printing paper, by a transfer unit provided near a process cartridge that includes a charging unit, a developing unit, and a cleaning unit.
In a visible-image processing step, two-component developer including toner and carrier and one-component developer not including carrier are used and two types of methods are used for applying the toner to the electrostatic latent image. One method is the contact type method by which these developers allow an electrostatic latent image on an image carrier to use the electrostatic force to attract toner. The other method is the non-contact type method by which toner is allowed to fly by an electric field action provided by a developing bias and the toner is attracted by an electrostatic latent image (see, for example, Japanese Patent Application Laid-Open No. 2003-270878).
When a charge amount of toner is balanced, in a developing process, with an electrostatic amount of an electrostatic latent image due to an electrostatic relation between the toner and the electrostatic latent image, no more toner is attracted by the electrostatic latent image and thus toner is not consumed any more (i.e., not used) and is recovered in a developing unit.
The toner recovered in the developing unit may cause a problem as described below. Specifically, toner is charged when the toner is stirred and mixed in a developing unit before being supplied to an electrostatic latent image or when friction is caused between a surface of a developing roller provided to be opposed to an image carrier in a developing unit and a blade provided near this surface. In a developing method using one component-system developer in particular, a toner layer may be allowed to be a uniform thin film by a surface of a developing roller composed of a metal roller for example to subsequently fly the toner toward an electrostatic latent image. In this case, the characteristic (charging characteristic in particular) of the toner may deteriorate due to friction caused in toner at the surface of the blade to cause a toner layer having a reduced thickness or of a developing roller or due to the stirring and mixing of the toner in transportation and stirring processes.
The deteriorated toner characteristic (deteriorated charging characteristic in particular) is caused when an external additive included in developer, which is an additive used to adjust the fluidity or an amount of friction charge and to improve a cleaning property, is buried in toner due to the temporal friction or stirring. An external additive may be the one that is composed of fine powders of colloidal silica (SiO2), titanium oxide, aluminum, or fatty acid metal to be added to the surface of toner.
When a developing concentration declines, toner may be supplied to provide an appropriate developing concentration. When new toner is introduced into a developing unit and is mixed with not-yet-consumed toner left in the developing unit (hereinafter, “residual toner”), friction may be caused between the former and the latter to charge the residual toner to have a reverse polarity. Then, the former toner may be bound with the latter toner and the bound toners may be attracted by a background potential of an image carrier, causing a dirty background. When the non-contact type developing method is used on the other hand, temporal friction or stirring of toner may increase a charge amount of the residual toner, which may suppress the flying performance when the toner flies while being attracted by the surface of a developing roller. This may cause a risk where an appropriate image concentration cannot be obtained.
A conventional method to prevent a declined image concentration due to deteriorated toner is disclosed in, for example, Japanese Patent No. 3029648. According to this method, a printing rate of every one image formation is calculated and, when the printing rate is smaller than a predetermined printing rate (i.e., when a toner consumption amount is small and an amount of residual toner is large), an image of a checkered pattern providing a toner consumption amount depending on the printing rate is formed before the next image formation is performed. Then, a transfer process is skipped and toner is recovered by a cleaning unit, thereby performing a forced consumption of toner.
Another example of a system for the forced consumption of toner is disclosed in, for example, Japanese Patent Application Laid-Open No. H9-34243. According to this system, an image forming proportion or the number of an output after the final consumption and supply of a predetermined amount of toner is determined to start a forced consumption mode when the image forming proportion is equal to or lower than a predetermined proportion.
However, this method has a problem as described below. In the control of toner supply based on the forced consumption of toner, toner is supplied only in an amount of forcedly-consumed toner. Thus, this control of toner supply does not consider the supply of toner in an amount of toner consumed for a formed image or supplies toner in an amount of toner consumed in a no-image part in parallel with the consumption.
When the method that does not consider the supply of toner in an amount consumed in an image-formed part is used, although the deterioration of developer can be prevented to some extent, the supply of toner is insufficient to cause an output image having a reduced density, causing an unstable control of an image concentration. Furthermore, the supply of toner in an amount of consumed toner in parallel with the consumption may not supply, when a no-image part has a small area, a sufficient amount of toner. This also causes, as in the above case, a problem of an unstable concentration of an output image.
A conventional toner supply includes a calculation of an amount of forcedly-consumed toner. In this calculation, whenever a paper is transferred, an amount of actually-consumed toner is compared with a predetermined toner consumption amount to supply toner. However, when an image having a very small image area ratio is suddenly outputted, an actual amount of toner for the image part is also small and thus a large amount of toner is not consumed and is used in a cleaning process. Thus, an increased load is applied to the cleaning process and thus a cleaning unit may not recover a part of the not-consumed toner, which may cause an abnormal image in which the not-recovered toner appears in the next image.
Recently, more image forming apparatuses use developer having a smaller particle diameter for the purpose of providing a high image quality. Although an improvement of a granular level by increasing an inclusion rate of fine powders having a small particle diameter is a very important factor to provide a high image quality, a combination of toner having a small particle diameter with carrier having a small particle diameter increases the bulk density of the toner attached to the carrier. Thus, a toner support amount and a charge amount of the carrier are increased and toner aggregation due to the small particle diameter is increased, thus deteriorating the fluidity.
As a result, the carrier and the toner having a contact to each other are difficult to separate from each other and thus deterioration of developer due to carrier spent is easily caused. In other words, developer having a particle diameter makes it difficult to control the supply of the developer in an appropriate manner.